anonstruct.c

#include <stdio.h>

struct Base {
	int foo;
	int bar;
};

struct Sub {
	struct Base;
	int baz;
	int qux;
};


int main(int argc, char * argv[]) {
	struct Sub sub;
	sub.foo = 1;
	sub.bar = 2;
	sub.baz = 3;
	sub.qux = 4;
	return 0;
}

gistfile1.txt

$ gcc -o test test.c
test.c:9:13: warning: declaration does not declare anything
    9 |  struct Base;
      |             ^
test.c: In function ‘main’:
test.c:17:5: error: ‘struct Sub’ has no member named ‘foo’
   17 |  sub.foo = 1;
      |     ^
test.c:18:6: error: ‘struct Sub’ has no member named ‘bar’; did you mean ‘baz’?
   18 |  sub.bar = 2;
      |      ^~~
      |      baz
$ gcc -fms-extensions -o test test.c
$